Fuel cell/battery hybrid system having battery charge-level control

ABSTRACT

A fuel cell/battery hybrid power system is disclosed which as a microprocessor based control system. The control system enables the fuel cell to be taken out of the system by a switch when its predetermined maximum desired energy output is about to be exceeded by load requirements. The batteries are protected from overdischarge through a switch and the surge battery is selected by the battery having the highest level of charge at any given time.

This is a continuation of copending application Ser. No. 191,718 filedon Mar. 2, 1988, which is a continuation of Ser. No. 048,996 filed May5, 1987 which is a continuation of Ser. No. 894,474 filed Aug. 4, 1986which is a continuation of Ser. No. 537,460 filed Sept. 29, 1983, allabandoned.

BACKGROUND OF THE INVENTION

This invention relates to hybrid power systems and, more particularly,to the control of such systems.

Cross reference is hereby made to two copending applications filed inthe U.S. Patent Office and assigned to the same assignee as thisapplication: application of I. Fekete entitled "Fuel Cell/Battery HybridSystem", Ser. No. 06/537,459, filed Sept. 29, 1983, now abandoned; andapplication of J. Early and J. Werth entitled "Fuel Cell/Battery ControlSystem", Ser. No. 06/537,461, filed Sept. 29, 1983, now abandoned.

In planning for future energy needs based on an evaluation of existingand potential economic and ecological problems, a great deal ofattention is being given to the development and utilization of energyefficient systems. Of particular interest are devices capable ofgenerating electricity by consuming plentiful or renewable fuels, theutilization of which minimizes environmental pollution which iseconomically justified. Fuel cells and gas turbines utilizing low weighthydrocarbons or hydrogen obtained by reforming methanol or similarorganic fuels represent likely candidates in this area. The fuel celltypically has a high efficiency and is largely without pollutingemissions. Noteworthy also is the versatility of the fuel cell inrespect to size and power level which is by adaptability to a modulardesign. Additionally, the fuel cell requires few moving parts, and is aquiet, reliable and comparatively maintenance free source of electricpower.

The above factors have made the fuel cell a likely candidate for use inspecialized vehicle transportation where needs are well defined andlong-standing. Unfortunately, fuel cells do not generally operate aswell as desired under conditions in which substantial variation in theload or surge demand are encountered. Fuel cell components can bedamaged by surge demands due to overheating, or the surge demand may notbe met at all. To overcome this problem, attempts have been made toutilize fuel cell stacks as primary energy sources in a so called"hybrid system", the purpose of which is to allow use of the majorenergy source (fuel cell) in a near optimum design by supplying only theaverage load requirements directly therefrom.

Such a system includes a storage source, such as a battery, to supplythe transient load increases and which can be recharged when the loaddrops below average. Thus, the load variation experienced by the primaryenergy source or fuel cell is reduced in frequency and magnitude. Thisallows for design and operation of the fuel cell to be within a bandrepresenting the average load rather than designing for peak load withdominant operation under peak load conditions.

Reduced emissions and increased efficiency are the expected benefits foroperating the hybrid system. A prior art type of hybrid power device isdisclosed in an article by J. B. O'Sullivan et al "Hybrid Power SourceFor Material Handling Equipment", IECEC 1975 Record, pp 229-236. In thesystem described by O'Sullivan, four fuel cell stacks are in parallelwith a battery sub-system. A controller controls the fuel cell andbattery sub-systems to limit the total current from the fuel cell tomaintain the fuel cell's life and to limit the fuel cell sub-systemvoltage as protection for the batteries. At low currents, the fuel cellsub-system voltage rises rapidly and fully charged batteries would bedriven into strong gasing if not protected. The O'Sullivan controlleroperates to vary a series resistance between the fuel cell stack and thebatteries, resulting in a variable voltage drop to maintain the voltageconstant at the battery at low currents and maintain the voltageconstant at the fuel cell at high currents.

Although the system described in O'Sullivan provides an approach tocontrol of the system, the degree of control may not be optimal sincesignificient resistive losses occur during operation. Further,O'Sullivan does not specify how the sharing of load current demand isprovided between the fuel cell stack and the batteries. Therefore, itappears that the O'Sullivan system does not provide a technique in whichthe peak surge energy demands in a hybrid system are compared to theaverage load requirements, and appropriate adjustments are made in thesystem, so that either the batteries alone or the fuel cell and thebattery are utilized in some combination to meet the load demand in amost efficient manner.

In addition to the O'Sullivan article, the following U.S. patents are ofgeneral interest in the fuel cell area: U.S. Pat. Nos. 4,000,003;3,883,368; 3,753,780; 3,546,020; and 3,473,337, and are mentioned hereinas a matter of general background.

Other fuel cell/battery hybrid systems of interest are disclosed in"Fuel Cell Systems for Vehicular Applications", SAE Technical PaperSeries, 800059, by Lynn, McCormick, Bobbett and Derouin, Feb. 25-29,1980; "A Fuel Cell-Battery Power Source for Electric Vehicles", TheFifth International Electric Vehicle Symposium, 782407(E), by Dowgiallo,Oct. 2-5, 1978; UK Patent Application GB No. 2 084 387 A, published onApr. 7, 1982; "An Assessment of the Status of Fuel Cell/Battery VehiclePower Systems", by Escher and Foster, February, 1980; Fuel-Cell-PoweredGolf Cart Report CONF-800523-1 of the Third International ElectricVehicle Exposition and Conference, St. Louis, Mo., by Bobbett,McCormick, Lynn, Kerwin and Derouin, May, 1980.

SUMMARY OF THE INVENTION

The present invention provides an improved hybrid power system whereinan energy storage means, such as batteries, and a primary energy source,such as a fuel cell stack, are selectively and automatically connectedin parallel circuitry through switching means to a common load, undercontrol of a microprocessor. The system is designed so that the poweroutput of the primary electrical energy source does not exceed a maximumdesign power output and the energy storage means may be controlled sothat it has a predetermined desired maximum and minimum level of chargemaintainable at all times. A controller regulates these aspects foroptimum use of the hybrid system.

In an embodiment of the invention, a load sensor is provided to monitorthe load current and sensors can be provided to measure the state of thecharge at each of the batteries. If the load sensor indicates a veryhigh load current, the microprocessor activates the appropriateswitching means such that both batteries are connected to the load withthe fuel cell stack disconnected. If the load sensor indicates a highcurrent demand, the microprocessor causes both batteries and the fuelcell stack to be connected into the load. If the load sensor indicates amedium current demand, the fuel cell stack and the battery which has ahigher state of charge is connected by the microprocessor to the load.Finally, if the load sensor indicates a low current load demand, thefuel cell and any of the batteries that are charged to a value less thana predetermined value are connected into the system. In essence, thesystem described herein manages energy transfers between the storagedevice and the primary energy source in an electric hybrid system, suchas a fuel cell-battery system.

In an embodiment of the invention, the batteries and the fuel cell stackare either connected to or disconnected from the common load in asequence determined by a microprocessor based on instantaneous loaddemand and battery state of charge conditions. As a result, system surgecapability is enhanced while peak power output of the primary energysource is controlled. Although the system is operable with a singlebattery, it is preferred in some embodiments that at least two or morebatteries be utilized since a multiple-battery system enhances systemreliability by avoiding the problem of a single bad cell in one of thebatteries causing the entire system to become inoperable. The conceptalso provides operational flexibility in that, while one battery isbeing cycled, the other battery equalizes and cools off. The system alsoenhances flexibility in system configuration and package design. Thisembodiment also provides means for enhancing the overall efficiency ofthe hybrid system since various energy source combinations are possibledepending on the battery charge level and the current load. Switchingbetween the power sources is arranged to provide for maximum efficiencyby allowing the fuel cell stack to operate close to its average ratedpower output for all load demand conditions. The fuel cell, whenconnected to the load, is also utilized to charge the batteries asappropriate, the shifting of load to the fuel cell stack minimizingbattery-run down by avoiding battery overcharge or overdischarge. Thisaspect extends battery life.

In an embodiment of the invention, energy storage means have a desiredminimum level of charge during operation. If it is a two battery hybridsystem, the surge battery is selected by the controller as the onehaving the highest level of charge at any given time.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the invention are explainedin the following description, taken in connection with the accompanyingdrawings wherein:

FIG. 1 is a schematic diagram of the hybrid system of the presentinvention;

FIG. 2 is a block diagram illustrating the use of a microprocessor tocontrol which power source or sources is to be utilized to supply thecommon load in the circuit of FIG. 1;

FIG. 3 is a flow chart depicting a typical program for operation of amicroprocessor in the system of FIG. 2; and

FIG. 4 is a schematic illustration of a single battery/fuel cell hybridcontrol system.

DETAILED DESCRIPTION

Referring now to FIG. 1, a schematic diagram of the hybrid system of thepresent invention is illustrated. The primary energy source 10 comprisesa fuel cell stack utilized in conjunction with two batteries, 12 and 14.Batteries 12 and 14 are generally rated in terms of open-circuitvoltage, ampere-hour capacity, and internal impedance. Both batteriesoperate as high power density storage devices and both are connected inparallel to fuel cell stack 10 in a manner to be described hereinafter.Although two batteries, 12 and 14, are preferably utilized in the systemdescribed, either a single battery or more than two batteries could alsobe utilized.

A load device 16, representing, for example, the current demand from amotorized vehicle, is connected in parallel across the energy sources10, 12 and 14 as illustrated. A plurality of relay contacts 18', 20' and22' are connected in series with energy sources 10, 12 and 14,respectively, as illustrated. As will be described in more detailhereinafter, if the contacts 18', 20' and 22' are caused to be in theirclosed position, the associated energy source is, in turn, connected tothe load 16. Current sensing devices 24 and 26, typically ammeters, areincluded in series with fuel cell 10 and load 16, respectively, asshown.

Charge sensors 28, and 30 are coupled in series with energy sources 12and 14, respectively, and function to sense the energy source chargelevel. Typically, such devices comprise an integrating coulombmeterwhich integrates the amperes in and out of the battery to track thetotal coulombs received and delivered by the battery. The charge sensormay incorporate a reset circuit to reset the coulombmeter periodicallyto compensate for drift in the coulombmeter. The coulombmeter can be aseparate unit within the system or its function could be incorporated inthe microprocessor described hereinafter.

FIG. 2 is a block diagram of the microprocessor based control systemused to determine which combinations of energy sources are to beconnected to the load 16. In particular, a programmable microprocessor40, such as the Zilog Z80 microprocessor chip, the operation of which isdescribed in Zilog Publication "Z80-CPU/Z80A-CPU Technical Manual",dated September, 1978 is responsive to the output from load sensor 26and charge sensors 28 and 30. Output signals from microprocessor 40 arecoupled to relays 18, 20 and 22 via leads 41, 42 and 43, respectively,relays 18, 20 and 22 controlling respective contacts 18', 20' and 22'shown in FIG. 1.

In essence, the output of load sensor 26 determines the operational modeof hybrid power system 8. As will be set forth in more detailhereinafter, four modes of operation selected corresponding to a veryhigh, high, medium, and low load currents

The qualitative definition of a low mode is one in which the entire loadcan be delivered by the fuel cell alone without help from any battery.The medium mode is where the fuel cell requires the assistance of atleast one battery to meet the load demand without the fuel cell beingoverloaded. The high mode is where the fuel cell and two batteries wouldbe required to meet the demand, one battery not being sufficient. Thevery high mode is where the fuel cell would have to be disconnectedbecause, even with both batteries assisting, the fuel cell would beoverloaded and subject to damage

In a representative 5 kilowatt system, for example, a low mode wouldcorrespond to a load rating not higher than approximately 6 kilowatts(e.g., 50 volts, 120 amperes); the medium mode would correspond to aload rating not higher than approximately 10 kilowatts (e.g., 200amperes); the high mode would correspond to a load rating not higherthan approximately 15 kilowatts (e.g., 300 amperes); and the very highmode would correspond to a load rating not higher than approximately 25kilowatts (e.g., 500 amperes).

It should be noted at this point that adapting a microprocessor, such asthe aforementioned Zilog Z80, to control various process operations suchas the switching sequences described herein, can be accomplished by oneof ordinary skill in the art having familiarity with the particularprocess requirements. It should also be noted that a circuit havingstandard analog or digital components could be utilized in lieu ofmicroprocessor 40 to provide the required switching sequence.

Referring back to FIG. 2, if the load sensor 26 determines that the loadcurrent is very high, microprocessor 40 generates signals on leads 41,42 and 43 such that contact 18' is opened and contacts 20' and 22' areclosed, to connect batteries 12 and 14 in parallel to the load 16 whilethe fuel cell is disconnected from load 16. If the load sensor 26detects a high load current, a signal is applied to relays 18, 20 and 22so that each of the appropriate contacts 18', 20' and 22' are closed,each of the energy sources 10, 12 and 14 therefor being connected inparallel to provide the current demand of load 16.

If the load sensor 26 detects a medium condition, microprocessor 40generates a signal on lead 41 such that relay 18 is activated, contact18' being closed thus connecting fuel cell stack 10 to load 16. In thiscase, the microprocessor 40 first senses the battery charge levels asdetermined by sensors 28 and 30. The microprocessor determines whetherbattery 12 or 14 has the higher state of charge and that battery isconnected by the microprocessor to the load 16 by energizing theappropriate relay. In other words, if sensor 28 indicates that battery12 has a higher state of charge than the charge of battery 14 asdetermined by sensor 30, relay 20 is activated such that correspondingcontact 20' (FIG. 1) is closed connecting battery 12 to load 16, whilecontact 22' remains open. If the charge level determination is reversed,relay contact 22' is closed and relay contact 20 is opened, connectingbattery 14 to load 16. In both these cases, relay contact 18' is closed,connecting fuel cell 10 to load 16.

Finally, if the load sensor 26 determines that a low load current isdemanded, relay 18 is activated, closing relay contact 18' and thusconnecting fuel cell 10 to load 16. Microprocessor 40 then determineswhich of the batteries 12 and 14 is charged to less than a predeterminedlevel, typically the 90% level. All those batteries which are less thanthat predetermined level are then connected into the circuit by closingthe appropriate contacts via relay 20 or 22 as required as needed.Specific variations of the above microprocessor control scheme are setforth hereinbelow with reference to Tables I-IV.

The present invention is further explained with reference to Table I inwhich the switching mode of the primary electrical energy source, suchas the fuel cell in this embodiment, and the first and second batteriesare shown with respect to four general load demand or mode conditions. Aminimum acceptable battery charge of 70% and a maximum of 90% isassumed, although variation in range is permissible, depending on theenvironment in which the system is used.

In Table I, at "Low" sensed current demand, for example, if bothbatteries are assumed to be at an arbitrary preset maximum 90% chargelevel, then no charge is imposed even though load demand on the primaryenergy source is less than the rated constant output of the primaryenergy source. It is noted that four possible battery configurations maybe provided.

Under "Medium" sensed load (Table I), the primary energy source pluseither battery 12 or 14 are available to meet a demand somewhatexceeding that provided by the primary electrical energy source. Thischoice may be mandated as battery 12 in one instance and battery 14 inthe other, depending on which is more highly charged. Such is then alsothe case in Tables II, III and IV, at a comparable demand level, sincethe battery of higher sensed potential, within the selected maximum andminimum range of 90%-70% charged level, is automatically selected as thesurge battery to supplement the primary electrical energy source, whilethe second battery remains in open (non-flow) bias.

Under "High" sensed energy demand (Table I), both batteries 12 and 14and the primary energy source are utilized to fully meet power demandson the system substantially in excess of the output of the primaryenergy source. The response is consistent at this surge demand (see alsoTables II, III) so long as the charge on each battery is greater than apreset 70% minimum charge level or other convenient minimum dependingupon use of the system. When both batteries are charged as shown inTable IV, neither battery would be connected when the load is low. Themicroprocessor is tied into all the switches to accomplish their changeof states when desired to function as intended

In all of the Tables used herein, the following legends are used:

O--Open Contact--no current flow

+--Closed Contact--Current flow in parallel

C--Charging Mode

                  TABLE I                                                         ______________________________________                                        SENSED                                                                        LOAD    FUEL CELL 10 BATTERY 12  BATTERY 14                                   ______________________________________                                        Very High                                                                             0            +           +                                            High    +            +           +                                            Medium  +            +           0                                                    +            0           +                                            Low     +            0           0                                                    +            0           C                                                    +            C           0                                                    +            C           C                                            ______________________________________                                    

Table II further exemplifies operation of the system of FIG. I when oneof the batteries 12 is at 90% charge and the other battery 14 at 85%.

                  TABLE II                                                        ______________________________________                                        SENSED               BATTERY 12  BATTERY 14                                   LOAD    FUEL CELL 10 (90% chg.)  (85% chg.)                                   ______________________________________                                        Very High                                                                             0            +           +                                            High    +            +           +                                            Medium  +            +           0                                            Low     +            0           C                                            ______________________________________                                    

Table III further exemplifies operation of the system of FIG. I whenboth batteries have a relatively low charge level but equal or exceed apreset 70% minimum charge level.

                  TABLE III                                                       ______________________________________                                        SENSED               BATTERY 12  BATTERY 14                                   LOAD    FUEL CELL 10 (70% chg.)  (75% chg.)                                   ______________________________________                                        Very    0            +           +                                            High                                                                          High    +            +           +                                            Medium  +            0           +                                            Low     +            C           0                                                    +            C           C                                            ______________________________________                                    

Table IV further exemplifies operation of the instant system whenbattery 14 is at a 90% charged level and battery 12 is also at a 90%charged level.

                  TABLE IV                                                        ______________________________________                                        SENSED               BATTERY 12  BATTERY 14                                   LOAD    FUEL CELL 10 (90% chg.)  (90% chg.)                                   ______________________________________                                        Very High                                                                             0            +           +                                            High    +            0           +                                            Medium  +            0           +                                            Low     +            0           0                                            ______________________________________                                    

The batteries 12 and 14 are recharged by the fuel cell stack 10. Inparticular, whenever the charge sensors 28 and 30 indicate that thestate of charge is greater than 90% during the operating part of thepower cycle, the batteries are prevented from continuing to charge andthus, are disconnected from fuel cell stack 10 under control ofmicroprocessor 40. Otherwise the batteries would be damaged. At the sametime, if the state of charge falls below some minimum state of charge;i.e., 70%, the battery would not be allowed to discharge any furtherunless needed to meet the load. It would be a very unusual condition foreither battery to be required to supply power and be 70% charged at thesame time if the system is designed appropriately.

It should be noted that the battery does not have to be disconnectedfrom the load in order to be recharged if the load is low. For example,if the state of charge of a battery is 80%, the battery is within thesafety limits and the battery can either charge or discharge. Therefore,the battery can be connected to both the load and to the fuel cell stackand if the load is light, the battery will accept charge. If the load isheavy, the battery is discharging. When the limits are reached, thebattery is connected at low current only if it needs to be charged or atmedium current only if it needs to be discharged.

In another similar embodiment of the system a single energy storagemeans, such as a battery, can be used in combination with switches and afuel cell to maintain an optimum hybrid system. In such an embodiment,the state of charge in the battery is continuously sensed and fed to thecontroller to determine if and when the battery is in danger of beingovercharged. The load current is also continuously monitored by thecontroller to determine which of three ranges of load conditions thesystem is operating under at any given instant. First, there is the lowrange within which the current is capable of being met by the fuel cellalone. Second, there is the medium range within which the load currentis capable of being met by the output of the fuel cell together with thebattery. Third, there is the very high range within which the loadcurrent is so high that even if the fuel cell and battery were togethersupplying current to the load, the current output of the fuel cell wouldhave to be more than its predetermined load rating. The predeterminedload rating of the fuel cell is designed into the system beforehand andis usually based on average load expected rather than all possible loadsurge levels.

The flowchart of FIG. 3, which shows a typical program for operation ofthe microprocessor 40, is abbreviated in that return blocks are omittedfor simplicity of illustration. As indicated elsewhere herein, suchprograms are well known and those skilled in the art will readilyrecognize the variations and additions which may be made to FIG. 3.

A schematic illustration of this embodiment of the simple, singlebattery/fuel cell system is shown in FIG. 4. Fuel cell 103 producescurrent as required by the load 105 and/or battery 101 for rechargingthe battery. Fuel cell 103 has a switch 104 which connects anddisconnects it from load 105 and battery 101. Battery 101 also has aswitch, switch 102, which connects and disconnects it from load 105 andfuel cell 103. The system also contains a suitable control means such ascontroller 106 which measures the current produced by the fuel cell 103via current sensor 107 and the current drawn by load 105 via currentsensor 108. From these two current values, the current flowing to andfrom the battery 101 can be calculated as well as the state of chargewithin the battery. The state of charge in battery 101, can becalculated by well known integration techniques, based on its currentflow, by the controller 106.

Within the range of operating conditions of the system described in FIG.4, switches 104 and 102 can produce a combination of switch conditionsto optimize the operation of the system. For instance, under conditionof low load and of battery 101 being in danger of overcharging, switch102 is disconnected or opened while switch 104 is connected or closed.The reason for this is to prevent the battery from being overcharged.If, conversely, switch 102 were allowed to be connected under theseconditions, that portion of the current flowing from the fuel cell whichwas not drawn by the load would flow into battery 101 thereby tending toovercharge it.

Under high load conditions in the system of FIG. 4, switches 102 and 104are both connected or closed. The reason for this is that the currentdrawn by the load is too high to be satisfied by the predesignedcapability of the fuel cell alone. Under very high load conditions ofthe system on FIG. 4, switch 104 is disconnected or opened and batteryswitch 102 is connected or closed. The reason for this is that if bothswitches were connected, the current requirements of the load wouldforce the fuel cell beyond its predetermined output capability.

The following technique may be used in bringing the fuel cell back online after very high load conditions occur. Such action can be carriedout by measuring the voltage of the load at the point that the fuel cellis disconnected therefrom under very high load conditions and storing itin the controller. After the very high load conditions pass and lowerload conditions are reinstated, such as high or low load conditionsreferred to above in FIG. 4, switch 104 should be connected to enablefuel cell 103 to produce current for load requirements. The precise timefor bringing the fuel cell 103 back into the system can be determined bysensing the load voltage continuously after very high load conditionsbegin, and closing switch 104 when the same sensed voltage reappears asthe very high load conditions dissipate. Alternatively, the load currentexisting just before or at the disconnect of switch 104 as very highload conditions appear can be sensed and placed into the memory ofcontroller 106 so that as load conditions are lowered below the veryhigh load conditons, as indicated by the load current being compared tothe level placed in the memory of the controller, switch 104 can againbe connected to bring the fuel cell 103 onto the system.

The following Tables V through VII depict the various conditions of thesystem elements similarly to the previous Tables. However, there areonly three operating load conditions instead of four; low, high and veryhigh, for this embodiment.

Table V shows a general case of the various possibilities in connectingthe fuel cell and battery under the three load range conditions of theembodiment in FIG. 4.

                  TABLE V                                                         ______________________________________                                        SENSED LOAD 105                                                                             FUEL CELL 103                                                                              BATTERY 101                                        ______________________________________                                        Very High     0            +                                                  High          +            +                                                  Low           +            0                                                                +            C                                                  ______________________________________                                    

Table VI shows the three operating load conditions when battery 101 is90% charged, or fully charged for this case.

                  TABLE VI                                                        ______________________________________                                                                   BATTERY 101                                        SENSED LOAD 105                                                                             FUEL CELL 103                                                                              (90% chg.)                                         ______________________________________                                        Very High     0            +                                                  High          +            +                                                  Low           +            0                                                  ______________________________________                                    

Table VII shows the three operating load conditions when battery 101 is80% charged and can be recharged during low load conditions.

                  TABLE VII                                                       ______________________________________                                                                   BATTERY 101                                        SENSED LOAD 105                                                                             FUEL CELL 103                                                                              (80% CHG.)                                         ______________________________________                                        Very High     0            +                                                  High          +            +                                                  Low           +            C                                                  ______________________________________                                    

Thus, it can be seen that the control system disclosed herein canutilize one, two or more batteries with the fuel cell. Each batteryutilized can have a switch associated therewith which can connect anddisconnect it to the system as dictated by the microprocessor. Thesystem may also have means to sense the fuel cell current, the loadcurrent and the state of charge in the batteries. The batteries may havea desired predetermined maximum and minimum state of charge duringoperation and one of the batteries, in a multiple battery system, may beused as the surge battery until it reaches its predetermined minimumcharge level. At such time another battery in the system of high chargelevel can replace it and the battery recharged by the fuel cell underlow load conditions. Also, in a two battery system, the surge batterycan be selected by whichever battery is at the highest charge level.

The present invention thus provides many advantages over the prior arthybrid power systems. In particular, a commercially availablemicroprocessor may be utilized to function with a proper combination ofenergy sources coupled into the load in a manner such that the loadcapacity of the fuel cell is not exceeded while the system surgecapability is enhanced. The above system protects the batteries againstovercharge and overdischarge and also protects the fuel cell fromoverloading by sensing the battery charge levels and the load current,respectively, and switching the batteries and the fuel cell to obtainthe best possible and most efficient system utilization.

It is to be understood that the above described embodiment of theinvention is illustrative only and that modifications thereof may occurto those skilled in the art. Accordingly, this invention is not to beregarded as limited to the embodiment disclosed herein, but is to belimited only as defined by the appended claims.

We claim:
 1. A hybrid power source system for supplying electricalcurrent to a load and for controlling the amount of current supplied inresponse to current demand from the load, the power source comprising:aprimary electrical energy source couplable to the load through a firstswitch means interposed therebetween and having a predetermined maximumdesired power output; first energy storage means couplable to the loadin parallel with the primary electrical power source through a secondswitch means interposed between the first energy storage means and theload; second energy storage means couplable to the load in parallel withboth the primary electrical power source and the first energy storagemeans through a third switch means interposed between the second energystorage means and the load, either said primary electrical energysource, said first and second energy storage means or combinationsthereof being connected in parallel to supply current to the loaddepending upon whether said first, said second or said third switchmeans or combinations thereof are appropriately energized; load currentsensing means for sensing the current flowing into the load; chargesensor means for sensing the state of charge in said first and secondenergy storage means and operatively associated with said control meansfor enabling selection by said control means among said first and secondenergy storage means of the energy storage means with the highest levelof charge, to serve as a surge energy storage means;and control meansresponsive to said load current sensing means for selectively energizingeither said first, said second or said third switch means in a mannersuch that the current demand on said primary electrical energy source ismaintained at or below approximately its predetermined maximum desiredpower output level.
 2. The system of claim 1 wherein the selection ofthe surge energy storage means changes as the state of charge of thesurge energy storage means previously selected drops below the state ofcharge of the other energy storage means.
 3. The system of claim 1wherein said primary electrical energy source recharges the energystorage means unselected as the surge energy storage means as neededduring periods when the load is not drawing the full output of saidprimary electrical energy source.
 4. The system of claim 1 wherein saidenergy storage means comprise storage batteries and said primary energysource comprises a fuel cell means.
 5. The system of claim 4 whereinsaid storage batteries are of identical capacity.
 6. The system of claim4 wherein the number of batteries is greater than two and each saidbattery is connected to the load in parallel with each of the otherbatteries and with the fuel cell means by a switch means interposedbetween its associated battery and the load for connecting anddisconnecting each said battery from the load.
 7. A hybrid power sourcesystem for supplying electrical current to a load and for controllingthe amount of current supplied in response to current demand from theload, the power source comprising:a primary electrical energy sourcecouplable to the load through a first switch means interposedtherebetween and having a predetermined maximum desired power output;first energy storage means couplable to the load in parallel with theprimary electrical power source through a second switch means interposedbetween the first energy storage means and the load; second energystorage means couplable to the load in parallel with both the primaryelectrical power source and the first energy storage means through athird switch means interposed between the second energy storage meansand the load, either said primary electrical energy source, or saidenergy storage means or combinations thereof being connected in parallelto supply current to the load depending upon whether said first, saidsecond or said third switch means or a combination thereof areappropriately energized; load current sensing means for sensing thecurrent flow to the load; charge sensor means for sensing the state ofcharge of said first and second energy storage means; and control meansresponsive to said load current sensing means and said charge sensormeans (i) for selectively energizing either said first, said second orsaid third switch means in a manner such that the current demand on saidprimary electrical energy source is maintained at or below approximatelyits predetermined maximum desired power output level, and (ii) forconnecting whichever one of said energy storage means has the higheststate of charge to the load when a surge in load current demand occurswhich can be satisfied by power from said primary electrical energysource and from only one of said energy storage means.